A central issue in second language (L2) acquisition concerns how explicit learning (EL) and implicit statistical learning (ISL) aptitudes contribute during the earliest stages of learning, and whether the effect of EL precedes that of ISL, as traditionally assumed, or can instead follow it. This paper explores these possibilities by tracking the contributions of these two aptitudes, as well as sustained attention (SA) to vocabulary and grammar learning across five sessions of exposure to an artificial language. Results indicated that vocabulary and grammar learning were modulated by EL and SA, with ISL additionally accounting for variance in vocabulary learning. Crucially, contrary to the standard view, for both grammar and vocabulary, the ISL effects were most pronounced early on, whereas the EL effects increased over time. These results underscore the dynamic interplay between the two aptitudes in early L2 acquisition and highlight the time-varying nature of their contributions.